gallivm: One code memory pool with deferred free.

gallivm: One code memory pool with deferred free.

Provide a JITMemoryManager derivative which puts all generated code into
one memory pool instead of creating a new one each time code is generated.
This saves significant memory per shader as the pool size is 512K and
a small shader occupies just several K.

This memory manager also defers freeing generated code until you tell
it to do so, making it possible to destroy the LLVM engine while keeping
the code, thus enabling future memory savings.

v2: Fix compilation errors with LLVM 3.4 (Jose)

Signed-off-by: José Fonseca <jfonseca@vmware.com>
Reviewed-by: Roland Scheidegger <sroland@vmware.com>